Copper produced by smelting is frequently electrolytically refined. For this reason, the molten copper product must be suitable for casting anodes. This makes it necessary to refine the copper to remove substantial quantities of dissolved sulphur and oxygen in order to cast desirable anodes. If the metal were cast directly, the high level of sulphur (typically 0.05%) and dissolved oxygen (typically 0.5%) would combine to form SO.sub.2 blisters in the cast metal. The purpose of in-line refining, as practiced with the present invention, is to remove sulphur from the molten copper. This is done in two steps: "blowing" and "poling". Blowing is the oxygenating step, wherein the sulphur is oxidized (to SO.sub.2), lowering the sulphur level (to typically 0.003%) in the molten copper. Poling--introducing hydrocarbon into the melt--minimizes the formation of copper oxide during solidification.
The present invention is directed to improvements in apparatus for the oxidation of molten copper. In one process for in-line copper refining, molten copper is subjected to oxygenation during the refining process.
At first, experiments were attempted with an oxygen lance similar to those used in the steel industry. Insufficient oxygenation resulted, due at least in part to the slag layer on the surface of the molten copper. The lance was incapable of penetrating the slag layer.
Another method of oxygenating the copper is by passing a stream of oxygen through a porous plug into the molten metal as shown, for example, by U.S. Pat. No. 3,904,180; No. 3,917,242; No. 3,972,709 or No. 4,277,381. An experimental arrangement was undertaken to determine the feasibility of using a porous plug to bubble oxygen onto the molten metal, as the metal moved past the porous plug by gravity. Bubbling pure oxygen through the porous plug, the plug failed prematurely. Two failure modes were noted; in the first, the pure oxygen reacted with the hot steel case of the porous plug and the plug shell melted and failed. In the second failure mode, molten copper peneterated the porous plug, and, combined with the first failure mode, molten copper leaked from the apparatus. It was found, however, that the plug did not fail if oxygen and nitrogen were mixed and bubbled through a copper sheathed porous plug fed with copper piping. Unfortunately, insufficient oxygenation took place.